The present invention relates to an image encoder, and more particularly, to a method and circuit for determining quantization intervals, taking full account of human visual perception characteristics.
A digital video signal, which includes much information, should be encoded for transmission through a transmission line of limited capacity. As efficient encoding methods for minimizing both degradation of image quality and information, there are known various transform encoding techniques such as differential pulse coded modulation (DPCM) for obtaining a difference image (using motion-estimation and motion-compensation of a video signal), and discrete cosine transform (DCT). Such techniques focus on minimization of redundancy in video signals and compressing information by quantization. Hence, in quantizing a video signal, it is very significant to minimize degradation of the quality of a restored image and to maximize information compression.
For both the minimization of quality degradation of a restored image and the maximization of information compression, there are two requirements.
First, human visual perception characteristics should be considered in determining optimized quantization intervals. That is, information of a video signal to which the human eye doesn't react sensitively is quantized at a large quantization interval and compressed to a large extent, while information of a video signal to which the human eye is sensitive is quantized at a small quantization interval and compressed to a small extent.
Second, a quantization interval should be determined so that the number of bits resulting from encoding is compatible with a given transmission speed.
In a conventional image encoding method, one frame is generally divided into 16.times.16 macroblocks, and a quantization interval for each block is determined. Generally, to determine the quantization interval, buffer requirements for a given transmission speed are calculated and a reference quantization interval is obtained in proportion to the calculated buffer requirements. A final quantization interval is determined by adjusting the reference quantization interval using the distribution value of a video signal within the macroblock. However, with respect to the conventional image encoding method, human visual perception characteristics are not sufficiently reflected and the number of bits resulting from encoding is not compatible with a transmission speed.